Few questions here:

1. How far does the cold ETC sensor advance the timing?
2. Would this amount of advance affect the MPG any?
3. Would there be any deterimental effects from this?

I was wondering about this wiring in a switch to fool the computer that it was cold if would make things a bit more efficient.

The ECT is a veriable resistor. it advances as it resists. As the engine warms the resistor resists less and the advance settles

I knew it wasn't a klixon switch, I was wondering about the effect of the advance on the engine and it's performance.

How far does it advance? I'm running stock timing, and no other performance mods, other than a fixed kitty.

It dependes on how cold it is. The extremety of an open circuit (PCM reads -40F) takes it to 14 degs act/des. What effect this has on MPG is probably minamal.

Degrees of timing still a bit over my head. Normal 190°F timing would be about what? Would it bring up the MPG? or mightin' I be gittin' 'er too toasty?

190 deg ECT would run ~4.0-5.0deg des/act timing at idle with stock -.5 tdc offset.

Not sure how much advance for a given temp, but it does advance timing, GMCTD & I experienced that last week during our testing 11 deg of timing at idle and ECT 160F, engine idle 750 rpm, once engine heated up ECT 175F idle down to 600 and timing 7.5 to 8 deg.

I've read about the different amounts of degrees of timing, but know little 'bout what ends up happening with it. I've heard of advancing timing, and retarding. Retarding I've heard of with running bio-diesel. So, what happens when you advance the timing?

I've seen the number of -1.9 come up a time or two, is that TDC offset, or is that timing, or what? Is that ECT sensor controled, or fixed value?

If I get this right TDC offset tells the computer where exactly the engine is in it's position, but the timing is the variable that gets changed by all the other "factors" in the equation?

TDC Offset is a fudge factor for hi-rpm timing, allowing for more performance than smog cops would allow.
It's been cleverly disguised in OBD-II, where OBD-I was show-and-tell.

That is what threw me off when TD was here - 'whur'd all yer data go, dude?'

Get an initial scan to verify +3.5deg base timing reference, rotate your Inj Pump about 2.5mm to driver's-side, do TDCOL procedure, verify -1.5deg TDCO, and you'll be ready to play with the big boys.
You'll still need a hot-chip re-flash, but it should run a little better.

Can also aggravate any already-in-progress lower-end failure - hence my suggestion to pull pan at next oil-change to see what you got.

I'm fairly certain ECT, IAT, and IP fuel temps are monitored and compared for out-of-limit variation, and will set a DTC.
You can 'switch' ECT for longer glows when cold, as in my earlier post, but running with extended offset will DTC.

I was thinking about a hybrid idea of QM's idea of the resistor and your idea of the switch. SPDT switch that will allow you to toggle over from the exsisting sensor to a variable pot to control the signal to the ECM. The pot could control what the computer thinks it's seeing, and adjust the timing advance accordingly. It's a cheapy charlie idea that I didn't know if it had any merit to it or not. Chips are for the kitchen cabinet here, not a budget line item here for family bus unfortunately.

The oil pan dropping is on the list to do too. All I need is the gasket, right?

Permatex Black RTV, lacquer thinner, the rear main cap rubber seal and dipstick tube o-ring - be sure to RTV the main-cap seal in the same areas as factory.
May be a paper seal between front pan rail and timing cover.
Pan rails get a healthy dose of RTV, full perimeter.

With a re-flash, increasing advance via IP tweak flattens and broadens the torque curve across 2000 - 2500rpm band, right where you need it.
Also increases hi-rpm hp minimally - only 3hp with the re-flash, but again broadens the curve.

Most folks thought the advance gave the stock prog some added zip.

BIG but, here- the IP\fuel distributor is rotated advanced for the -1.94deg, where idle is +8.5deg, cruise is ~17deg.

ECT allows ~+12deg at idle with the distributor in stock +3.5deg location, also allowing ~+12deg for cruise when engine is at operating temps.
Advance falls back toward +3.5deg under load, as in up a hill.

Not sure how cold-advance alone will run, with IP at +3.5deg base timing.
Can't hurt anything, but result will not be same as IP tweak.

You could try the variable ECT thing - DTC resets itself when prob goes away - see what happens.

QM posted the temp\resistance chart awhile back - a 10-turn pot with the decade-dial should allow some 'metered' control.

Make sure your harmonic balancer and accessory drive pulley is good, and the bottom end is not damaged.